The invention relates to a method for operating a direct injection diesel engine which is operated in a first operating region corresponding to low to medium load in such a way that combustion of the fuel takes place at a local temperature below the formation temperature of NOx and with a local air ratio above the ratio at which particulates are formed, where fuel injection is initiated at a crank angle of 50° to 5° before top dead center of the compression phase and where exhaust gas is recirculated at a recirculation rate of 50% to 70%. The invention furthermore relates to an internal combustion engine implementing the method.
The most important variables governing the combustion process in a combustion engine with internal combustion are the phase of the combustion or rather the phase of the start of combustion, the maximum rate of increase of cylinder pressure, and the peak cylinder pressure.
In an internal combustion engine in which combustion occurs essentially by self-ignition of a directly injected volume of fuel, the governing variables are largely determined by injection timing, charge composition, and ignition lag. These parameters in turn depend on a multitude of variables, such as engine speed, amount of fuel, intake temperature, charge pressure, effective compression ratio, amount of inert gas in the cylinder charge, and temperature of the various parts of the engine.
Increasingly strict legal requirements necessitate the development of novel conceptions in combustion design, in order to reduce the emission of particulates and NOx in diesel engines.
It is known that NOx and particulate emission in the exhaust gas may be reduced by increasing the ignition lag by advancing the start of injection, such that combustion occurs by self-ignition of a lean fuel-air mixture. A variant of this type is termed the HCLI-method (Homogeneous Charge Late Injection). In a combustion process of this type fuel injection takes place at a large enough distance from top dead center of the compression phase to give rise to a largely homogeneous fuel-air mixture. By means of exhaust gas recirculation the combustion temperature may be kept below the temperature required for NOx-formation. Since the homogenization of fuel and air is time-dependent, this method is limited as regards engine speed and load, and particle emission will increase if homogenization is insufficient.